In recent years, as the development of power generation systems such as photovoltaics and fuel cell systems has advanced and made off-grid power generation available, more interest has been focused on power storage devices to temporarily store privately generated electricity.
A power storage device is structured to have a secondary battery as the storage battery and to have its own charging device. The power storage device is capable of storing electricity generated by charging the secondary battery through its own charging device. When requested by an external load, the electricity charged in the secondary battery is transmitted to the external load.
For a safe charging procedure, the end-of-charge voltage is set in a secondary battery at the maximum value for charged voltage. If the voltage rises beyond the end-of-charge voltage, internal active-material layers of electrodes fall into a state of overcharge. When a secondary battery is overcharged, acceleration in damage to the secondary battery material and electrolyte decomposition on the material surfaces is known to occur, thereby causing rapid deterioration of the secondary battery. Accordingly, to achieve a long lifetime of a secondary battery, it is necessary to prevent overcharging the battery and to suppress the acceleration in material damage and electrolyte decomposition.
Meanwhile, to store electricity generated by off-grid power generation systems or the like, a relatively larger capacity and power output are required. To increase the capacity and power output, multiple secondary batteries may be connected in series to form a battery pack.
When a battery pack is used, overcharging is usually controlled pack by pack, not battery by battery within a pack. As a result, depending on the degree of deterioration or other variations between each individual battery, a certain secondary battery in the battery pack may be overcharged when charge control is performed pack by pack; in other words, controlling overcharge of a certain secondary battery in the pack is something that is yet to be solved. Therefore, when a battery pack is formed by connecting multiple secondary batteries in series, it is necessary to use secondary batteries with excellent overcharge resistance.
Considering the above, to prevent overcharge of a certain secondary battery, Patent Literature 1 proposes a battery pack structured to have multiple batteries connected in series, including a battery possessing an inflection region for detecting the depth of charge set in advance at a desired level and other batteries that are not intended for detecting the depth of charge. According to the battery pack described in Patent Literature 1, when the voltage of the battery for detecting the depth of charge reaches a specified voltage, the depth of charge of the entire battery pack is accurately detected, allowing the pack to be used within a voltage range that is kept below the overcharge.